The Palestinian delegates to the United Nations are within their rights to push a Security Council resolution condemning Israeli settlement activity in the West Bank (or any resolution at all), and the United States is within its rights to veto such a resolution.
The United States may no longer be the indispensable nation in every corner of the globe, but in the Middle East it still is. If there is one nation that can influence Israel more effectively through direct engagement, it is the United States. This has been a major consideration behind countless U.S. vetoes against Security Council resolutions that were one-sided and/or politically motivated. What will be gained by changing course at this sensitive juncture?
The United States and Israel are strategic allies, and the UN is certainly no stranger to friends standing up for friends... and far worse. Violators of international law -- even outright genocide -- are routinely exempted from UN condemnation, despite often strong efforts by the United States and others. If the UN were known for acting on principle in every instance and nations voted on the basis of international law rather than specific relationships and strategies, the world would be a safer and better place. But that's just not the case.
Except for Canada, perhaps, every government in the world comes to the UN to take care of itself first. The United States has also been in the top tier of those willing to rise above specific economic and strategic national interest.
Cherry-picking the enforcement of "international law" will not secure Palestinians a viable, sovereign state. Now may not be the right time for meaningful negotiations with Israel, whether for outside reasons, internal Palestinian politics, or Israel's right-wing government and frustrated public. So it may be wise for Palestinians and others to pile on against Israel at this time and call for selectively enforcing international law. But the United States is not compelled to participate in such parlor games.
There is a long tradition of Palestinians bringing their case to the United Nations, where they get an automatic majority in the General Assembly. The UNGA even adopted a resolution 35 years ago declaring Zionism to be a form of racism -- in other words, even the most hateful and ridiculous proposition can succeed at the UN, if it has the right sponsors. When the Security Council does adopt a resolution, largely due to the threat of a U.S. veto, they often impact positively on conflict situations in a way that UNGA resolutions cannot. This is not one of those constructive resolutions.
Palestinians are openly campaigning for recognition of statehood, in lieu of difficult negotiations. Their strategy within the UN is to gain footholds and build momentum. Palestinians and the Arab League states would portray U.S. support for this seemingly limited resolution to rally Western nations to formally debate Palestinian statehood. This will probably happen in the coming weeks anyway, but a change in U.S. veto practices would significantly boost the legitimacy and destructiveness of that effort.
But when there remain avenues for negotiation between the parties, appealing to the UN only when convenient has no credibility, and should not be mistaken for a turn toward peace. Israelis should also understand that allowing settlements to proceed makes little sense, now or in the future. But they need a reason to care, and this will not provide one.
If supporting one politicized resolution in a politicized forum would substantively advance the prospects for peace, the United States -- unlike most member states -- would be likely to do so. But this is probably not such a time, and this is definitely not the resolution that will do so.